Fixed focus lens

ABSTRACT

A fixed focus lens comprising, sequentially from an object side, a first lens group having a positive refractive power; a second lens group having a negative refractive power; and a third lens group having a positive refractive power. The first lens group includes an aperture stop. The second lens group is configured by a single lens element. During focusing, the second lens group moves along an optical axis and the first lens group and the third lens group are fixed with respect to an imaging plane.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a fixed focus lens used in 35 mmcameras, video cameras, digital still cameras, and the like.

2. Description of the Related Art

To make a captured image and the viewfinder image coincide, single lensreflex cameras are equipped with a mechanism by which light that haspassed through the imaging lens is reflected by a mirror placed in frontof the film and the light is guided to an optical viewfinder.Consequently, fixed focus lenses used in single lens reflex cameras musthave a long back focus, limiting degrees of freedom in terms of design.Digital cameras, on the other hand, merely have to display the capturedimage on an electronic viewfinder to accomplish the same thing asconventional single lens reflex cameras. Therefore, by omitting theoptical viewfinder and the mirror for guiding light to form the image atthe optical viewfinder, a compact apparatus is realized, the so-called“mirror-less single lens camera” has been introduced. In mirror-lesssingle lens cameras, the back focus can be shortened, thereby affordingthe advantage of improved degrees of freedom in the design of the fixedfocus lens used in these cameras. Consequently, there are also a largenumber of fixed focus lenses that can be mounted to mirror-less singlelens cameras (see, for example, Japanese Patent Nos. 3950571 and3445554, and Japanese Patent Application No. 2003-43348).

The optical system disclosed in Japanese Patent No. 3950571 achievessimplification in that the focusing lens group is configured by onenegative lens, nonetheless, configurations of other lens groups includenumerous lenses and do not facilitate simplification. Further, althoughthe shortening of focusing stroke is considered, with the focusing lensgroup disposed farther on the object side than the diaphragm, the frontelement diameter becomes larger consequent to the position of theentrance pupil becoming deep. For these reasons the optical systemdisclosed in Japanese Patent No. 3950571 does not sufficiently achievereductions in size and is not suitable for recent mirror-less cameras ofwhich further size reductions are demanded.

The optical system disclosed in Japanese Patent No. 3445554 achieves asimpler configuration than the optical system disclosed in JapanesePatent No. 3950571, but with respect to the focusing stroke, thefocusing sensitivity of the image focusing is small and consequently,when images are captured at the minimum object distance, the focusinggroup has to be moved greatly and thus, a reduction in the size of theoptical system cannot be achieved.

The optical system disclosed in Japanese Patent Application Laid-OpenPublication No. 2003-43348 facilitates simplification by a two-lensconfiguration of the focusing group, but here again, since the focusingsensitivity is small, when images are captured at the minimum objectdistance, the focusing group has to be moved greatly and thus, areduction in the size of the optical system cannot be achieved.

Thus, conventional fixed focus lenses, such as those recited in thepatent documents above do not achieve sufficient size and weightreductions.

SUMMARY OF THE INVENTION

It is an object of the present invention to at least solve the aboveproblems in the conventional technologies.

A fixed focus lens according to one aspect of the invention includes,sequentially from an object side, a first lens group having a positiverefractive power; a second lens group having a negative refractivepower; and a third lens group having a positive refractive power. Thefirst lens group includes an aperture stop. The second lens group isconfigured by a single lens element. During focusing, the second lensgroup moves along an optical axis and the first lens group and the thirdlens group are fixed with respect to an imaging plane.

The other objects, features, and advantages of the present invention arespecifically set forth in or will become apparent from the followingdetailed description of the invention when read in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view (along the optical axis) of a fixedfocus lens according a first example;

FIG. 2 is a diagram of various types of aberration of the fixed focuslens according the first example, at infinity focus;

FIG. 3 is a diagram of various types of aberration of the fixed focuslens according the first example, at the focus state for a magnificationof 0.025×;

FIG. 4 is a diagram of various types of aberration of the fixed focuslens according the first example, at the focus state for the minimumobject distance;

FIG. 5 is a cross sectional view (along the optical axis) of the fixedfocus lens according a second example;

FIG. 6 is a diagram of various types of aberration of the fixed focuslens according the second example, at infinity focus;

FIG. 7 is a diagram of various types of aberration of the fixed focuslens according the second example, at the focus state for amagnification of 0.025×;

FIG. 8 is a diagram of various types of aberration of the fixed focuslens according the second example, at the focus state for the minimumobject distance;

FIG. 9 is a cross sectional view (along the optical axis) of the fixedfocus lens according a third example;

FIG. 10 is a diagram of various types of aberration of the fixed focuslens according the third example, at infinity focus;

FIG. 11 is a diagram of various types of aberration of the fixed focuslens according the third example, at the focus state for a magnificationof 0.025×;

FIG. 12 is a diagram of various types of aberration of the fixed focuslens according the third example, at the focus state for the minimumobject distance;

FIG. 13 is a cross sectional view (along the optical axis) of the fixedfocus lens according a fourth example;

FIG. 14 is a diagram of various types of aberration of the fixed focuslens according the fourth example, at infinity focus;

FIG. 15 is a diagram of various types of aberration of the fixed focuslens according the fourth example, at the focus state for amagnification of 0.025×; and

FIG. 16 is a diagram of various types of aberration of the fixed focuslens according the fourth example, at the focus state for the minimumobject distance.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the accompanying drawings, exemplary embodiments accordingto the present invention are explained in detail below.

A fixed focus lens according to the present invention includessequentially from the object side, a first lens group having a positiverefractive power, a second lens group having a negative refractivepower, and a third lens group having a positive refractive power.

To reduce the overall length of the optical system, the combinedmagnification of the second and the third lens groups, which aresubsequent the first lens group, is preferably increased and the focallength of the first lens group is shortened. However, if themagnification of the second lens group and the third lens group isunnecessarily large, aberration occurring at the first lens group isintensified at subsequent groups. To prevent this negative effect, thefirst lens group has to minimize the aberration that occurs therein. Tofavorably correct aberration by the first lens group, the first lensgroup includes at least two positive lenses and negative lenses,respectively. Thus, in the fixed focus lens according to the presentinvention, the first lens group includes at least two positive lensesand at least two negative lenses. The first lens group is fixed withrespect to the imaging plane, even during focusing.

Further, in the fixed focus lens according to the present invention, anaperture stop is disposed within the first lens group. Preferably, atthe focal length of a 35 mm equivalent focal length camera, in astandard lens range in the neighborhood of 35 mm to 50 mm, the firstlens group is separated into a front group and a rear group, betweenwhich an aperture stop is disposed. By symmetrically disposing thelenses with respect to the aperture stop, various types of aberrationcan be appropriately corrected. On the other hand, for use at the focallength of a 35 mm equivalent focal length camera, in an intermediatetelephoto range in the neighborhood of 90 mm to 150 mm, the aperturestop does not necessarily have to be disposed in an intermediateposition between the front group and the rear group. In this case, theaperture stop may be disposed nearer to the imaging plane in the firstlens group. A negative lens is disposed adjacent to aperture stop, onthe imaging plane side. By such a configuration, in particular,favorable correction of sagittal image plane curvature can be expected.Further, by disposing the aperture stop within the first lens group,which is farther on the object side than the second lens group, not onlyare various types of aberration appropriately corrected, but the frontelement diameter can be reduced, which is useful.

The second lens group preferably includes a single lens element.Focusing is performed by moving the second lens group in a directionalong the optical axis. By reducing the weight of the focusing group,the load on the autofocus mechanism can be reduced, enabling reducedpower consumption and a smaller outer diameter of the barrel. A singlelens element includes a single ground lens, an aspheric lens, a compoundaspheric lens, and a cemented lens, but does not include non-cementedlenses having a space therebetween, such as two positive lenses.

The third lens group is fixed with respect to the imaging plane.Although the third lens group can be moveable, the third lens group ispreferably fixed to prevent damage of the lens support mechanism by theinsertion of, for example, a finger from outside barrel.

By the above configuration, the fixed focus lens according to thepresent invention is configured by fewer lens elements, enabling weightreductions and shortening of the overall optical system length. Further,disposal the aperture stop at an appropriate position within the firstlens group brings about favorable correction of various types ofaberration and enables the front element diameter of the optical systemto be decreased.

One object of the present invention is to provide a fixed focus lensthat is compact, light-weight and has an inner focusing system offavorable imaging performance. To further ensure this object isachieved, in addition to the configuration above, various conditionsindicated below are further set.

The following conditional expression is preferably satisfied, where β2Gis paraxial magnification of the second lens group, in a state ofinfinity focus and β3G is paraxial magnification of the third lensgroup, in a state of infinity focus, in the fixed focus lens.0.7<|(1−β2G)×β3G|<7.0  (1)

Conditional expression (1) determines the stroke amount of the focusinggroup from the state of infinity focus to the focus state for theminimum object distance, and prescribes focusing sensitivity of theoptical system. The value prescribed by conditional expression (1) is animportant element that determines the size and imaging performance ofthe optical system. Below the lower limit of conditional expression (1),the stroke amount of the focusing group increases and when the focusstate for the desired minimum object distance is established, increasedoverall optical length becomes unavoidable and therefore, undesirable.On the other hand, above the upper limit of conditional expression (1),the magnification of the second lens group and subsequent groupsincreases, which particularly invites deterioration of off-axis,chromatic difference of magnification, whereby the number of lenselements included in the first lens group has to be increased to preventthis adverse state, making a simple configuration of the optical systemdifficult and therefore, undesirable.

If conditional expression (1) is further within the following range,more favorable effects can be expected.0.8<|(1−β2G)×β3G|<6.5  (1)′

By satisfying the range prescribed by conditional expression (1)′, ashorter overall length of the optical system can be achieved along withimproved imaging performance.

If conditional expression (1)′ is further within the following range,more favorable effects can be expected.0.9<|(1−β2G)×β3G|<6.0  (1)″

By satisfying the range prescribed by conditional expression (1)″, aneven shorter overall length of the optical system can be achieved alongwith further improved imaging performance.

To simplify the configuration of the optical system, in the fixed focuslens according to the present invention, the third lens group isconfigured by one positive lens, or a positive lens and a negative lens.The third lens group has to have a positive refractive power to increasethe angle of incidence of the chief ray to the imaging plane. Thus, thefixed focus lens according to the present invention preferably satisfiesthe following conditional expression, where F3 is the focal length ofthe third lens group and F is the focal length of the entire opticalsystem.0.4<F3/F<1.8  (2)

Conditional expression (2) expresses a condition for achieving bothshortening of the overall length of the optical system and maintainingimaging performance. Below the lower limit of conditional expression(2), the back focus of the optical system becomes short and the focusingsensitivity cannot be increased, whereby increases in the overall lengthof the optical system become unavoidable and thus, is not desirable. Onthe other hand, above the upper limit of conditional expression (2),although the focusing sensitivity can be increased, the back focus ofthe optical system becomes too long and thus, is not desirable.

If conditional expression (2) is further within the following range,more favorable effects can be expected.0.45<F3/F<1.65  (2)′

By satisfying the range prescribed by conditional expression (2)′, ashorter overall length of the optical system can be achieved along withfurther improved imaging performance.

If conditional expression (2)′ is further within the following range,more favorable effects can be expected. By satisfying the rangespecified by the conditional expression (2)″, an even shorter overalllength of the optical system can be achieved along with further improvedimaging performance.0.5<F3/F<1.5  (2)′

In the focus lens according to the present invention, the surface on theimaging plane side of the second lens group plays a role in correctingunderside image plane curvature occurring at the first and the secondlens groups, to the over-side. Thus, the fixed focus lens according tothe present invention preferably satisfies the following conditionalexpression, where FR2 is the radius of curvature of the surface on theimaging plane side of the second lens group, and F is the focal lengthof the entire optical system.0.2<FR2/F<0.6  (3)

Conditional expression (3) prescribes the radius of curvature of thesurface on the imaging plane side of the second lens group, to maintainimaging performance. Below the lower limit of conditional expression(3), image plane curvature is corrected to the over-side excessively,making it difficult to balance axial and off-axis imaging performance.Above the upper limit of conditional expression (3), conversely, theimaging plane curvature becomes too far on the underside andparticularly at the minimum object distance, the imaging plane curvaturebecomes great, degrading imaging performance in the focus state for theminimum object distance and thus, is undesirable.

If conditional expression (3) is further within the following range,more favorable effects can be expected.0.25<FR2/F<0.55  (3)′

By satisfying the range prescribed by conditional expression (3)′,without sacrificing size reductions of the optical system, morefavorable imaging performance can be obtained.

If conditional expression (3)′ is further within the following range,even more favorable effects can be expected.0.30<FR2/F<0.5  (3)

By satisfying the range prescribed by conditional expression (3)″,without sacrificing size reductions of the optical system, furtherimprovements in imaging performance can be expected.

The fixed focus lens according to the present invention preferablysatisfies the following conditional expressions, where nd is the averagerefractive index at d-line of the second lens group and υd is theaverage Abbe number at d-line of the second lens group.1.50<nd<2.0  (4)25<υd<68  (5)

Conditional expression (4) prescribes an average refractive index atd-line of the second lens group and conditional expression (5)prescribes an average Abbe number at d-line of the second lens group.Inclusion of a lens having a high refractive index in the second lensgroup is advantageous because the aberration can be kept to a minimumand the focusing sensitivity can be increased. Inclusion of a highdispersion lens in the second lens group is effective in correctingchromatic aberration occurring at the first lens group, particularlychromatic aberration of magnification. Here, deviating from theconditions prescribed by conditional expressions (4) and (5) andconfiguring the second lens group of a lens having a low refractiveindex and low dispersion, results in an inability to correct at thesecond lens group, the aberration that could not be completely correctedat the first lens group.

If conditional expressions (4) and (5) are further within the followingranges, more favorable effects can be expected.1.53<nd<1.95  (4)′30<υd<65  (5)′

By satisfying the ranges prescribed by conditional expressions (4)′ and(5)′, more favorable effects can be expected and without sacrificingsize reductions of the optical system, more favorable aberrationcorrection can be realized.

If conditional expression (4)′ and (5)′ further satisfy the followingranges, more favorable effects can be expected.1.55<nd<1.90  (4)″31<υd<65  (5)″

By satisfying the ranges prescribed by conditional expressions (4)″ and(5)″, without sacrificing size reductions of the optical system, morefavorable aberration correction can be realized.

Average refractive index is, for example, the average value of thematerials from which the compound aspheric lens, etc. is made, formingthe single lens element. Similarly, the average Abbe number is, forexample, the average value of the materials from which the compoundaspheric lens, etc. is made, forming the single lens element. Further,in the fixed focus lens according to the present invention, the focallength of the first lens group is greatly affected by the overall lengthof the optical system. To shorten the overall length of the opticalsystem, the focal length of the first lens group has to be appropriatelyset. Thus, the fixed focus lens according to the present inventionpreferentially satisfies the following condition, where F1 is the focallength of the first lens group and F is the focal length of the entireoptical system.0.3<F1/F<1.1  (6)

Conditional expression (6) prescribes a focal length of the first lensgroup. Below the lower limit of conditional expression (6), themagnification of lens groups subsequent to the first lens groupincreases, a large number of lens are required, and the realization ofan optical system having favorable imaging performance with fewer lensesbecomes difficult. On the other hand, above the upper limit ofconditional expression (6), the overall length of the optical systemincreases, sacrificing size reductions of the optical system.

If conditional expression (6) is further within the following range,more favorable effects can be expected.0.35<F1/F<1.0  (6)′

By satisfying the range prescribed by conditional expression (6)', highperformance can be maintained while realizing a simple, compact fixedfocus lens.

If conditional expression (6)′ is further within the following range,more favorable effects can be expected.0.40<F1/F<0.9  (6)″

By satisfying the range prescribed by conditional expression (6)″, highoptical performance can be maintained while realizing a simpler and morecompact fixed focus lens.

As described, the fixed focus lens according to the present inventionhas few lenses and thus, is light-weight and enables a shortening of theoverall length of the optical system. By disposing the aperture stop atan appropriate position within the first lens group, various types ofaberration can be favorably corrected and the front element diameter ofthe optical system can be reduced. Further, by satisfying each of theconditional expressions above, a more compact and light-weight fixedfocus lens can be realized, having an inner focusing system of favorableimaging performance.

With reference to the accompanying drawings, examples of the fixed focuslens according to the present invention will be described. The inventionis not limited by the examples below.

FIG. 1 is a cross sectional view (along the optical axis) of the fixedfocus lens according a first example. The fixed focus lens includessequentially from an object side (object not depicted), a first lensgroup G₁₁ having a positive refractive power, a second lens group G₁₂having a negative refractive power, and a third lens group G₁₃ having apositive refractive power. At an imaging plane IMG at the rear (rightside in the drawing) of the third lens group G₁₃, the light receivingsurface of an imaging sensor such as a CCD, CMOS, etc. is disposed.

The first lens group G₁₁ includes sequentially from the object side, apositive lens L₁₁₁, a positive lens L₁₁₂, a negative lens L₁₁₃, anegative lens L₁₁₄, and a positive lens L₁₁₅. Between the negative lensL₁₁₃ and the negative lens L₁₁₄, an aperture stop ST, which prescribes agiven aperture, is disposed. The surface on the object side of thenegative lens L₁₁₄ is aspheric. The first lens group G₁₁ is fixed anddoes not move during focusing.

The second lens group G₁₂ is configured by a negative lens L₁₂₁. Thesurface on the imaging plane IMG side of the negative lens L₁₂₁ isaspheric. The second lens group G₁₂ moves along the optical axis, fromthe object side toward the imaging plane IMG side, whereby focusing isperformed from a state of infinity focus to the focus state for theminimum object distance.

The third lens group G₁₃ is configured by a positive lens L₁₃₁. Thethird lens group G₁₃ is also fixed and does not move during focusing.

Here, various values related to the fixed focus lens according the firstexample are given.

(Lens data) r₀ = ∞ (object surface) d₀ = D(0) r₁ = 56.088 d₁ = 2.941 nd₁= 1.91082 νd₁ = 35.2 r₂ = −78.854 d₂ = 0.200 r₃ = 16.767 d₃ = 2.831 nd₂= 1.91082 νd₂ = 35.2 r₄ = 60.401 d₄ = 0.879 r₅ = −153.141 d₅ = 0.800 nd₃= 1.84666 νd₃ = 23.8 r₆ = 13.386 d₆ = 3.013 r₇ = ∞ (aperture stop) d₇ =1.600 r₈ = −123.515 (aspheric surface) d₈ = 0.800 nd₄ = 1.68893 νd₄ =31.2 r₉ = 66.188 d₉ = 5.540 r₁₀ = 48.146 d₁₀ = 2.497 nd₅ = 1.91082 νd₅ =35.2 r₁₁ = −30.362 d₁₁ = D(11) r₁₂ = 69.746 d₁₂ = 0.800 nd₆ = 1.68893νd₆ = 31.2 r₁₃ = 12.168 (aspheric surface) d₁₃ = D(13) r₁₄ = 18.127 d₁₄= 2.638 nd₇ = 1.72916 νd₇ = 54.7 r₁₅ = 145.590 d₁₅ = FB r₁₆ = ∞ (imagingplane) (Constant of the cone (K) and aspheric coefficients (A₄, A₆, A₈,A₁₀)) (eighth plane) K = 0, A₄ = 1.29983 × 10⁻⁷, A₆ = 8.66172 × 10⁻⁸, A₈= −1.05350 × 10⁻⁹, A₁₀ = 1.64719 × 10⁻¹¹ (thirteenth plane) K = 0, A₄ =−1.93195 × 10⁻⁵, A₆ = −2.22932 × 10⁻⁷, A₈ = 1.22482 × 10⁻⁹, A₁₀ =−3.13255 × 10⁻¹¹ (Values for focusing states) Minimum object Infinity0.025x distance (0.089x) Focal length of entire 35.270 34.883 33.704optical system (F) Focal length of first 27.950 lens group G₁₁ (F1)Focal length of third 28.150 lens group G₁₃ (F3) F no. 2.00 2.01 2.06Half angle of view (ω) 13.56 13.36 12.85 Image height 8.50 8.50 8.50Length of optical 47.343 47.360 47.414 system D(0) ∞ 1414.116 401.999D(11) 1.500 2.083 3.5945 D(13) 3.795 3.212 1.700 FB (back focus) 17.5017.50 17.50 (Values related to conditional expression (1)) Paraxialmagnification of second lens group G₁₂, at infinity focus (β2G) = 4.00Paraxial magnification of third lens group G₁₃, at infinity focus (β3G)= 0.32 |(1 − β2G) × β3G| = 0.96 (Values related to conditionalexpression (2)) F3/F = 0.80 (Values related to conditional expression(3)) FR2/F = 0.35 (Values related to conditional expression (4)) nd =1.68893 (Values related to conditional expression (5)) νd = 31.2 (Valuesrelated to conditional expression (6)) F1/F = 0.80

FIG. 2 is a diagram of various types of aberration of the fixed focuslens according the first example, at infinity focus. FIG. 3 is a diagramof various types of aberration of the fixed focus lens according thefirst example, at the focus state for a magnification of 0.025×. FIG. 4is a diagram of various types of aberration of the fixed focus lensaccording the first example, at the focus state for the minimum objectdistance. In the drawings, g indicates the wavelength aberrationcorresponding to g-line (λ=435.83 nm), d indicates the wavelengthaberration corresponding to d-line (λ=587.56 nm). Furthermore, in thedrawings, s and m shown with respect to astigmatism, respectivelyindicate aberration at the sagittal image plane and at the meridonalimage plane.

FIG. 5 is a cross sectional view (along the optical axis) of the fixedfocus lens according a second example. The fixed focus lens includessequentially from the object side (object not depicted), a first lensgroup G₂₁ having a positive refractive power, a second lens group G₂₂having a negative refractive power, and a third lens group G₂₃ having apositive refractive power. At the imaging plane IMG at the rear (rightside in the drawing) of the third lens group G₂₃, the light receivingsurface of an imaging sensor such as a CCD, CMOS, etc. is disposed.

The first lens group G₂₁ includes sequentially from the object side, apositive lens L₂₁₁, a positive lens L₂₁₂, a negative lens L₂₁₃, anegative lens L₂₁₄, and a positive lens L₂₁₅. Between the negative lensL₂₁₃ and the negative lens L₂₁₄, the aperture stop ST, which prescribesa given aperture, is disposed. The surface on the object side of thenegative lens L₂₁₁ is aspheric. The first lens group G₂₁ is fixed anddoes not move during focusing.

The second lens group G₂₂ is configured by a negative lens L₂₂₁. Thesurface on the imaging plane IMG side of the negative lens L₂₂₁ isaspheric. The second lens group G₂₂ moves along the optical axis, fromthe object side toward the imaging plane IMG side, whereby focusing isperformed from a state of infinity focus to the focus state for theminimum object distance.

The third lens group G₂₃ is configured by a positive lens L₂₃₁. Thethird lens group G₂₃ is also fixed and does not move during focusing.

Here, various values related to the fixed focus lens according thesecond example are given.

(Lens data) r₀ = ∞ (object surface) d₀ = D(0) r₁ = 23.220 d₁ = 4.219 nd₁= 1.88300 νd₁ = 40.8 r₂ = 218.633 d₂ = 0.200 r₃ = 17.836 d₃ = 2.517 nd₂= 1.72916 νd₂ = 54.7 r₄ = 35.075 d₄ = 1.608 r₅ = 682.870 d₅ = 0.800 nd₃= 1.80518 νd₃ = 25.5 r₆ = 12.539 d₆ = 3.317 r₇ = ∞ (aperture stop) d₇ =1.600 r₈ = 6515.162 (aspheric surface) d₈ = 0.800 nd₄ = 1.68893 νd₄ =31.2 r₉ = 41.134 d₉ = 2.209 r₁₀ = 24.428 d₁₀ = 2.603 nd₅ = 1.91082 νd₅ =35.2 r₁₁ = −97.665 d₁₁ = D(11) r₁₂ = −391.408 d₁₂ = 0.800 nd₆ = 1.56732νd₆ = 42.8 r₁₃ = 12.687 (aspheric surface) d₁₃ = D(13) r₁₄ = 21.841 d₁₄= 3.552 nd₇ = 1.62041 νd₇ = 60.3 r₁₅ = −53.109 d₁₅ = FB r₁₆ = ∞ (imagingplane) (Constant of the cone (K) and aspheric coefficients (A₄, A₆, A₈,A₁₀)) (eighth plane) K = 0, A₄ = 1.31522 × 10⁻⁶, A₆ = 4.08403 × 10⁻⁸, A₈= 3.73283 × 10⁻¹⁰, A₁₀ = 2.41864 × 10⁻¹² (thirteenth plane) K = 0, A₄ =−2.17308 × 10⁻⁵, A₆ = −3.37294 × 10⁻⁷, A₈ = 4.64174 × 10⁻⁹, A₁₀ =−6.19872 × 10⁻¹¹ (Values for focusing states) Minimum object Infinity0.025x distance (0.105x) Focal length of entire 40.00 39.84 38.72optical system (F) Focal length of first 33.64 lens group G₂₁ (F1) Focallength of third 25.41 lens group G₂₃ (F3) F no. 2.00 2.04 2.16 Halfangle of view (ω) 12.09 11.77 10.85 Image height 8.50 8.50 8.50 Lengthof optical 49.337 49.346 49.395 system D(0) ∞ 1617.695 400.000 D(11)1.500 2.225 4.572 D(13) 4.773 4.048 1.700 FB (back focus) 18.85 18.8518.85 (Values related to conditional expression (1)) Paraxialmagnification of second lens group G₂₂, at infinity focus (β2G) = 6.06Paraxial magnification of third lens group G₂₃, at infinity focus (β3G)= 0.20 |(1 − β2G) × β3G| = 1.01 (Values related to conditionalexpression (2)) F3/F = 0.63 (Values related to conditional expression(3)) FR2/F = 0.32 (Values related to conditional expression (4)) nd =1.56732 (Values related to conditional expression (5)) νd = 42.8 (Valuesrelated to conditional expression (6)) F1/F = 0.84

FIG. 6 is a diagram of various types of aberration of the fixed focuslens according the second example, at infinity focus. FIG. 7 is adiagram of various types of aberration of the fixed focus lens accordingthe second example, at the focus state for a magnification of 0.025×.FIG. 8 is a diagram of various types of aberration of the fixed focuslens according the second example, at the focus state for the minimumobject distance. In the drawings, g indicates the wavelength aberrationcorresponding to g-line (λ=435.83 nm), d indicates the wavelengthaberration corresponding to d-line (λ=587.56 nm). Furthermore, in thedrawings, s and m shown with respect to astigmatism, respectivelyindicate aberration at the sagittal image plane and at the meridonalimage plane.

FIG. 9 is a cross sectional view (along the optical axis) of the fixedfocus lens according a third example. The fixed focus lens includessequentially from the object side (object not depicted), a first lensgroup G₃₁ having a positive refractive power, a second lens group G₃₂having a negative refractive power, and a third lens group G₃₃ having apositive refractive power. At the imaging plane IMG at the rear (rightside in the drawing) of the third lens group G₃₃, the light receivingsurface of an imaging sensor such as a CCD, CMOS, etc. is disposed.

The first lens group G₃₁ includes sequentially from the object side, apositive lens L₃₁₁, a positive lens L₃₁₂, a negative lens L₃₁₃, anegative lens L₃₁₄, and a positive lens L₃₁₅. The positive lens L₃₁₂ andthe negative lens L₃₁₃ are cemented. Between the negative lens L₃₁₃ andthe negative lens L₃₁₄, the aperture stop ST, which prescribes a givenaperture, is disposed. The surface on the object side of the positivelens L₃₁₂ and both sides of the negative lens L₃₁₄ are aspheric,respectively. The first lens group L₃₁ is fixed and does not move duringfocusing.

The second lens group G₃₂ is configured by a negative lens L₃₂₁. Thesurface on the imaging plane IMG side of the negative lens L₃₂₁ isaspheric. The second lens group G₃₂ moves along the optical axis, fromthe object side toward the imaging plane IMG side, whereby focusing isperformed from a state of infinity focus to the focus state for theminimum object distance.

The third lens group G₃₃ is configured by a positive lens L₃₃₁. Thethird lens group G₃₃ is also fixed and does not move during focusing.

Here, various values related to the fixed focus lens according the thirdexample are given.

(Lens data) r₀ = ∞ (object surface) d₀ = D(0) r₁ = 284.746 d₁ = 2.440nd₁ = 1.91082 νd₁ = 35.2 r₂ = −84.740 d₂ = 0.150 r₃ = 11.199 (asphericsurface) d₃ = 4.358 nd₂ = 1.91082 νd₂ = 35.2 r₄ = 22.008 d₄ = 0.800 nd₃= 2.00069 νd₃ = 25.5 r₅ = 8.198 d₅ = 4.000 r₆ = ∞ (aperture stop) d₆ =3.095 r₇ = −8.357 (aspheric surface) d₇ = 1.000 nd₄ = 1.84666 νd₄ = 23.8r₈ = −11.150 (aspheric surface) d₈ = 0.300 r₉ = 22.517 d₉ = 3.902 nd₅ =1.91082 νd₅ = 35.2 r₁₀ = −29.820 d₁₀ = D(10) r₁₁ = −79.568 d₁₁ = 0.800nd₆ = 1.84666 νd₆ = 23.8 r₁₂ = 14.000 (aspheric surface) d₁₂ = D(12) r₁₃= 32.737 d₁₃ = 4.253 nd₇ = 1.88300 νd₇ = 40.8 r₁₄ = −25.745 d₁₄ = FB r₁₅= ∞ (imaging plane) (Constant of the cone (K) and aspheric coefficients(A₄, A₆, A₈, A₁₀)) (third plane) K = 0, A₄ = −3.66301 × 10⁻⁶, A₆ =−1.21276 × 10⁻⁷, A₈ = 1.69648 × 10⁻⁹, A₁₀ = −1.79277 × 10⁻¹¹ (seventhplane) K = 0, A₄ = 3.562023 × 10⁻⁴, A₆ = 1.26121 × 10⁻⁶, A₈ = 3.15187 ×10⁻⁸, A₁₀ = −1.50996 × 10⁻¹¹ (eighth plane) K = 0, A₄ = 2.66056 × 10⁻⁴,A₆ = 5.09367 × 10⁻⁸, A₈ = 2.99061 × 10⁻⁸, A₁₀ = −3.38903 × 10⁻¹⁰(twelfth plane) K = 0, A₄ = 1.13628 × 10⁻⁵, A₆ = −5.64832 × 10⁻⁷, A₈ =1.193354 × 10⁻⁹, A₁₀ = 3.02064 × 10⁻¹¹ (Values for focusing states)Minimum object Infinity 0.025 distance (0.072x) Focal length of entire28.33 28.40 28.36 optical system (F) Focal length of first 23.47 lensgroup G₃₁ (F1) Focal length of third 16.90 lens group G₃₃ (F3) F no. 2.02.0 2.0 Half angle of view (ω) 16.76 16.32 15.55 Image height 8.50 8.508.50 Length of optical 45.5 45.5 45.5 system D(0) ∞ 1143 404 D(10) 1.501.97 2.84 D(12) 3.04 2.57 1.70 FB (back focus) 15.8 15.8 15.8 (Valuesrelated to conditional expression (1)) Paraxial magnification of secondlens group G₃₂, at infinity focus (β2G) = 293.3 Paraxial magnificationof third lens group G₃₃, at infinity focus (β3G) = 0.004 |(1 − β2G) ×β3G| = 1.17 (Values related to conditional expression (2)) F3/F = 0.60(Values related to conditional expression (3)) FR2/F = 0.49 (Valuesrelated to conditional expression (4)) nd = 1.84663 (Values related toconditional expression (5)) νd = 23.8 (Values related to conditionalexpression (6)) F1/F = 0.83

FIG. 10 is a diagram of various types of aberration of the fixed focuslens according the third example, at infinity focus. FIG. 11 is adiagram of various types of aberration of the fixed focus lens accordingthe third example, at the focus state for a magnification of 0.025×.FIG. 12 is a diagram of various types of aberration of the fixed focuslens according the third example, at the focus state for the minimumobject distance. In the drawings, g indicates wavelength aberrationcorresponding to g-line (λ=435.83 nm), d indicates the wavelengthaberration corresponding to d-line (λ=587.56 nm). Furthermore, in thedrawings, s and m shown with respect to astigmatism, respectivelyindicate aberration at the sagittal image plane and at the meridonalimage plane.

FIG. 13 is a cross sectional view (along the optical axis) of the fixedfocus lens according a fourth example. The fixed focus lens includessequentially from the object side (object not depicted), a first lensgroup G₄₁ having a positive refractive power, a second lens group G₄₂having a negative refractive power, and a third lens group G₄₃ having apositive refractive power. At the imaging plane IMG at the rear (rightside in the drawing) of the third lens group G₄₃, the light receivingsurface of an imaging sensor such as a CCD, CMOS, etc. is disposed.

The first lens group G₄₁ includes sequentially from the object side, apositive lens L₄₁₁, a positive lens L₄₁₂, a negative lens L₄₁₃, anegative lens L₄₁₄, and a positive lens L₄₁₅. Between the negative lensL₄₁₃ and the negative lens L₄₁₄, the aperture stop ST, which prescribesa given aperture, is disposed. The surface on the object side of thenegative lens L₄₁₄ is aspheric. The first lens group G₄₁ is fixed anddoes not move during focusing.

The second lens group G₄₂ is configured by a negative lens L₄₂₁. Thesurface on the imaging plane IMG side of the negative lens L₄₂₁ isaspheric. The second lens group G₄₂ moves along the optical axis, fromthe object side toward the imaging plane IMG side, whereby focusing isperformed from a state of infinity focus to the focus state for theminimum object distance.

The third lens group G₄₃ includes sequentially from the object side, apositive lens L₄₃₁ and a negative lens L₄₃₂. The third lens group G₄₃ isalso fixed and does not move during focusing.

Here, various values related to the fixed focus lens according thefourth example are given.

(Lens data) r₀ = ∞ (object surface) d₀ = D(0) r₁ = 24.947 d₁ = 4.087 nd₁= 1.88300 νd₁ = 40.8 r₂ = 2036.994 d₂ = 0.200 r₃ = 17.575 d₃ = 2.647 nd₂= 1.72916 νd₂ = 54.7 r₄ = 41.758 d₄ = 1.252 r₅ = −626.969 d₅ = 0.800 nd₃= 1.80518 νd₃ = 25.5 r₆ = 13.096 d₆ = 3.118 r₇ = ∞ (aperture stop) d₇ =1.600 r₈ = −185.378 (aspheric surface) d₈ = 0.800 nd₄ = 1.68893 νd₄ =31.2 r₉ = 45.523 d₉ = 2.663 r₁₀ = 28.713 d₁₀ = 2.468 nd₅ = 1.90366 νd₅ =31.3 r₁₁ = −62.409 d₁₁ = D(11) r₁₂ = −313.795 d₁₂ = 0.800 nd₆ = 1.56732νd₆ = 42.8 r₁₃ = 12.347 (aspheric surface) d₁₃ = D(13) r₁₄ = 19.939 d₁₄= 4.372 nd₇ = 1.62041 νd₇ = 60.3 r₁₅ = −30.346 d₁₅ = 0.977 r₁₆ = −24.000d₁₆ = 0.800 nd₈ = 1.90270 νd₈ = 31.0 r₁₇ = −33.209 d₁₇ = FB r₁₈ = ∞(imaging plane) (Constant of the cone (K) and aspheric coefficients (A₄,A₆, A₈, A₁₀)) (eighth plane) K = 0, A₄ = 1.59362 × 10⁻⁶, A₆ = 7.92123 ×10⁻⁸, A₈ = −2.39664 × 10⁻¹⁰, A₁₀ = 5.01713 × 10⁻¹² (thirteenth plane) K= 0, A₄ = −3.05079 × 10⁻⁵, A₆ = −1.28827 × 10⁻⁷, A₈ = −3.92826 × 10⁻⁹,A₁₀ = 3.01816 × 10⁻¹¹ (Values for focusing states) Minimum objectInfinity 0.025x distance (0.1x) Focal length of entire 38.53 38.36 37.31optical system (F) Focal length of first 32.29 lens group G₄₁ (F1) Focallength of third 24.63 lens group G₄₃ (F3) F no. 2.0 2.0 2.0 Half angleof view (ω) 16.0 15.5 14.2 Image height 8.5 8.5 8.5 Length of optical48.5 48.5 48.5 system D(0) ∞ 1557 401 D(11) 1.50 2.20 4.32 D(13) 4.523.83 1.70 FB (back focus) 15.9 15.9 15.9 (Values related to conditionalexpression (1)) Paraxial magnification of second lens group G₄₂, atinfinity focus (β2G) = 5.58 Paraxial magnification of third lens groupG₄₃, at infinity focus (β3G) = 0.21 |(1 − β2G) × β3G| = 0.98 (Valuesrelated to conditional expression (2)) F3/F = 0.70 (Values related toconditional expression (3)) FR2/F = 0.35 (Values related to conditionalexpression (4)) nd = 1.56732 (Values related to conditional expression(5)) νd = 42.8 (Values related to conditional expression (6)) F1/F =0.92

FIG. 14 is a diagram of various types of aberration of the fixed focuslens according the fourth example, at infinity focus. FIG. 15 is adiagram of various types of aberration of the fixed focus lens accordingthe fourth example, at the focus state for a magnification of 0.025×.FIG. 16 is a diagram of various types of aberration of the fixed focuslens according the fourth example, at the focus state for the minimumobject distance. In the drawings, g indicates the wavelength aberrationcorresponding to g-line (λ=435.83 nm), d indicates the wavelengthaberration corresponding to d-line (λ=587.56 nm). Furthermore, in thedrawings, s and m shown with respect to astigmatism, respectivelyindicate aberration at the sagittal image plane and at the meridonalimage plane.

Among the values for each of the examples above, r₁, r₂, . . . indicateradii of curvature for each lens, diaphragm surface, etc.; d₁, d₂, . . .indicate the thickness of the lenses, diaphragm, etc. or the distancebetween surfaces thereof; nd₁, nd₂, . . . indicate the refraction indexof each lens with respect to the d-line (λ=587.56 nm); and υd₁, υd₂, . .. indicate the Abbe number with respect to the d-line (λ=587.56 nm) ofeach lens. Lengths are indicated in units of [mm] and angles areindicated in [degrees].

Each aspheric surface shape above is expressed by equation [1], where Zis the depth of the aspheric surface, c is curvature (1/r), h is theheight from the optical axis, the travel direction of light is positive,K is the constant of the cone, A₄, A₆, A₈, and A₁₀ are the fourth,sixth, eighth, and tenth aspheric coefficients.Z=ch ²/[1+{1−(1+K)c ² h ²}^(1/2) ]+A ₄ h ⁴ +A ₆ h ⁶ +A ₈ h ⁸ +A ₁₀ h¹⁰  [1]

As described, the fixed focus lens according to each of the examples isconfigured by fewer lens elements, enabling weight reductions and ashortening of the length of optical system. Further, the aperture stopis disposed at an appropriate position within the first lens group,whereby various types of aberration can be favorably corrected and thefront element diameter of the optical system can be reduced. Bysatisfying the conditional expressions above, a more compact,light-weight fixed focus lens having an inner focusing system offavorable imaging performance can be realized. The fixed focus lensaccording to the examples employ lenses and cemented lenses ofappropriately formed aspheric surfaces, thereby enabling favorableoptical performance to be maintained with fewer lens elements.

As described, the fixed focus lens according to the present invention isapplicable to 35 mm cameras, video cameras, electronic still cameras,etc. and is particularly suitable for mirror-less single lens camerashaving a short back focus.

Although the invention has been described with respect to a specificembodiment for a complete and clear disclosure, the appended claims arenot to be thus limited but are to be construed as embodying allmodifications and alternative constructions that may occur to oneskilled in the art which fairly fall within the basic teaching hereinset forth.

The present document incorporates by reference the entire contents ofJapanese priority document, 2010-288863 filed in Japan on Dec. 24, 2010.

What is claimed is:
 1. A fixed focus lens comprising, sequentially froman object side, a first lens group having a positive refractive power; asecond lens group having a negative refractive power; and a third lensgroup having a positive refractive power, wherein the first lens groupincludes an aperture stop, the second lens group is configured by asingle lens element, during focusing, the second lens group moves alongan optical axis and the first lens group and the third lens group arefixed with respect to an imaging plane, a conditional expression (6)0.3<F1/F<1.1 is satisfied, where F1 is a focal length of the first lensgroup and F is a focal length of the entire optical system, and aconditional expression (1) 0.7<|(1−β2G)×β3G|<7.0 is satisfied, where β2Gis paraxial magnification of the second lens group, at infinity focusand β3G is paraxial magnification of the third lens group, at infinityfocus.
 2. The fixed focus lens according to claim 1, wherein the thirdlens group is configured by one positive lens, or a positive lens and anegative lens, and satisfies a conditional expression (2) 0.4<F3/F<1.8,where F3 is a focal length of the third lens group and F is a focallength of the entire optical system.
 3. The fixed focus lens accordingto claim 2, wherein a conditional expression (3) 0.2<FR2/F<0.6 issatisfied, where FR2 is radius of curvature of a surface on an imagingplane side of the second lens group and F is a focal length of theentire optical system.
 4. The fixed focus lens according to claim 3,wherein conditional expressions (4) 1.50<nd<2.0 and (5) 25<υd<68 aresatisfied, where nd is average refractive index at d-line of the secondlens group and υd is average Abbe number at d-line of the second lensgroup.